The use of carbon molecular sieves to separate various materials has been known for several decades. Walker in "Molecular Sieves" Mineral Industries (January 1966) discloses obtaining carbon molecular sieves by charring polyvinylidine chloride and vinyl chloride-vinylidine chloride copolymer. These chars are said to have large micropores and are useful for the separation of hydrocarbons. The sieves can be modified by combining the char with a thermosetting furan resin or lignite pitch and carbonizing the product.
U.S. Pat. No. 3,801,513, Munzner, et al., (1974) states that it was not known that carbon molecular sieves (CMS) could be suitable for the separation of oxygen and nitrogen. This reference describes obtaining CMS for oxygen separation by treating coke having volatile components of up to 5% with a carbonaceous substance which splits off carbon at 600.degree. to 900.degree. C., thereby narrowing the pores present in the coke. It is stated that the average pore size of the adsorbent must be below 3 angstroms to effect oxygen separation from nitrogen. The average pore diameter can be adjusted by changing the intensity of the treatment. Coconut shell coke is a suitable starting material, among others. A preference is stated for a particle size in the range of 0.1 to 20 millimeters and suitable carbonaceous substances which can be used in the treatment include benzene, ethylene, ethane, hexane, cyclohexane, methanol, and the like. It is stated that if coking is carried with pitch, bitumin, tar or tar oil, gaseous coking materials are formed in the heat treatment and this atmosphere can be adjusted to exert the desired effect. In this way, introduction of at least part of the carbonaceous substance is avoided, but addition of the carbonaceous substance can modify the coke to increase its separating capacity.
Japanese Publication No. Sho 49-37036 (1974) describes making a carbon molecular sieve by condensing or polymerizing a phenol resin or furan resin so that the resin is adsorbed on a carbon adsorbent and thereafter carbonizing the product by heating. Mixtures of the resins can also be used. The resin forming material is dissolved in water, methanol, benzene or creosote oil and the solution is used to impregnate the carbon adsorbent. Carbonizing can be carried out at 400.degree. to 1,000.degree. C. in an inert gas. This operation is said to reduce the pore diameter of the carbon adsorbent.
Nakano et al., "Control of Micropores of Molecular Sieving Carbon by Impregnation of Hydrocarbons and Heat Treatment", presented at the 20th Spring Chemical-Industrial Convention at Hirneji, October (1987) describe modification of molecular sieving carbon having micropores less than 5 angstroms in diameter by impregnation with hydrocarbon mixtures and thereafter heat treating at 750.degree. to 950.degree. C. in order to control micropore diameter to 2.8 to 4 angstroms and make the CMS suitable for separating oxygen and nitrogen. Granulated carbon was formed from coal tar or coal tar pitch and coconut char. The liquid hydrocarbons used for impregnating the carbon were mixtures of napthalene with coal tar, diphenyl or fluorene in various concentrations from 5 to 50 wt. %. Fluorene was found to have the greatest effect on oxygen and nitrogen adsorption rates.
Chihara et al., Proc. Third Pacific Chem. Eng. Congress, Vol. 1 (1983) discloses that CMS which is a pelletized granular activated carbon can be treated by thermally decomposing benzene in a fluidized bed of the CMS to deposit carbon thereon and thereby adjust the overall mass transfer coefficients of oxygen and nitrogen in the CMS. A nitrogen product gas of 99.5% purity was obtained by pressure swing adsorption. A constant adsorption capacity was interpreted as indicating carbon deposition at the mouth of the micropore.
U.S. Pat. No. 4,458,022, Ohsaki et al., (1984) refers to several prior art processes for narrowing the micropores of active carbon by precipitating soot in the micropores and describes a method said to provide improved selectivity for separating nitrogen from air. The method involved using coconut shell charcoal and coal tar binder, acid washing, adding coal tar and heating to 950.degree. to 1,000.degree. C. for 10 to 60 minutes. The coal tar is said to penetrate into the surface of the active carbon and decompose to grow carbon crystallite on the inner surface of the micropore. It is stated that for PSA separation of nitrogen and oxygen, the oxygen adsorption capacity should be more than 5 milliliters per gram and the selectivity more than 22 to 23.
Japanese Patent Application No. Sho 62-176908 (1987) discloses a method for making carbon molecular sieves suitable for separating oxygen and nitrogen involving the use of carbon from coconut shells and coal tar or coal tar pitch binder to form particles which are dry distilled at 600.degree. to 900.degree. C., washed with mineral acid and water and dried, and then impregnated with creosote, 2,3-dimethylnapthalene, 2,4-xylenol or quinoline and heat treated for 10 to 60 minutes at 600.degree. to 900.degree. C. in inert gas. Both oxygen adsorption rate and selectivity are said to be improved and the procedure is said to be superior to the use of hydrocarbons, such as benzene, pyrolyzed in the gas phase so that carbon produced adheres to the carbonaceous surface.
Surinova, Khim. Tevrd. Top., Moscow (5) 86-90 (1988) describes obtaining carbon molecular sieves for concentration of nitrogen from air by carbonizing coals using benzene vapor and inert gas. The treatment had no effect on macropores but the pyrocarbon formed on decomposition of benzene is said to block the micropore inlets. Although this reference contains some recognition of the relationship between the hydrocarbon size and the pore size of the carbon, the work was apparently unsuccessful in reducing the concept to practice on coal samples whose capacity had been increased by oxidative treatment.
Hoffman, et al., "Pyrolysis of Propylene Over Carbon Active Sites II. Pyrolysis Products", Carbon Vol. 26, No. 4, pages 485-499 (1988) describe depositing carbon on graphitized carbon black by thermal decomposition of propylene, recognizing that propylene is excluded from some of the carbon sites. Both oxidized and unoxidized carbon samples were studied.
None of the above references describe a procedure which is suitable for modifying carbon molecular sieves which have a large population of relatively small micropores, on the order of 4.5 to 8 angstroms, but are still too large for effective separation of gases such as oxygen and nitrogen. In order to be effective for air separation by pressure swing adsorption (PSA) the adsorbent must not only exhibit good selectivity as suggested by the '022 patent, but must also have a high adsorbing capacity and permit high rates of gas throughput. While it is known that CMS having micropores on the order of .about.4 angstroms can be used successfully for this purpose, these adsorbents are very expensive and it is highly desirable to find a method of modifying a less expensive CMS, such as one having a characteristic pore size of about 5 angstroms, by a simple straight-forward procedure.
Although air separation can be effected over carbon molecular sieve adsorbents (CMS's), these adsorbents separate oxygen from air on a kinetic basis, sorbing the smaller oxygen molecules rapidly relative to the slightly larger nitrogen molecules. In order to effect separation the adsorbent must have pore openings of about the molecular diameter of the larger gas in the mixture (nitrogen in air). This allows for rapid adsorption of the smaller component and slower diffusion of the larger component, resulting in high kinetic selectivity. The ability to control the size of the pore openings on a CMS to exacting specifications, to tenths of an angstrom in the case of air separation, is a major challenge for preparing CMS adsorbents. Improved CMS adsorbents are needed to reduce the cost of air separation by pressure swing adsorption (PSA) systems since the adsorbent is a key part of the performance of the entire process unit.